Insulating coating for magnetic steel



United States Patent 3,138,492 INSULATING CGATING FOR MAGNETIC STEEL Paul E. Perry, Tarentum, and Clarence L. Miller, Jr., Pittsburgh, Pa., and Victor J. Piasecki, Haclrettstown, N.J., assignors to Allegheny Ludlum Steel Corporation, Brackenridge, Pa., a corporation of Pennsylvania No Drawing. Filed Oct. 11, 1961, Ser. No. 144,298 4 Claims. (Cl. 148-6.15)

This invention relates to the production of electrical grades of fiat rolled silicon steel, and relates in particular to the application of an improved insulating coating for electrical grades of silicon steel strip or sheet.

In the manufacture of electrical grades of steel, such as silicon steel strip for winding magnetic cores, it is necessary to provide an adequate insulating film to the surface of the strip. Such electrical grades of silicon steel strip are conventionally alternately cold rolled and heat treated to obtain directional magnetic properties. Final cold rolling generally provides a strip material of thin gauge, usually about 14 mils in thickness. The final product is frequently coated with a thin coating of electrical resistant material which may be of an organic nature such as resins, varnishes or lacquers, or more commonly of an inorganic nature such as sodium silicate, talc, magnesium oxide, borax, and more recently, phosphates. The coated strip is conventionally employed for winding in the direction of rolling, which is also the direction of improved magnetic properties, into oval or circular shaped cores for various applications. The insulating coating serves to separate the alternate steel layers from one another and thus effectively reduce core loss when electrical flux is induced into the magnetic core by reducing eddy currents. The thickness of the insulating film is relatively critical for a given application in that if the film is too thick it reduces the quantity of steel present and available in a given size core to provide the desired electrical induction intended. However, if on the other hand the film is too thin, eddy currents will create high core loss.

Phosphate coatings such as are taught by US. Patents 2,492,095 and 2,501,846 to Gifford have proved to be satisfactory When employed as insulating coatings for the oriented materials described above. These steels must be subjected to long-time (up to 60 hours), high temperature (up to 2300 F.) box anneals to develop the desired and required grain orientation and anisotropic magnetic properties. It is necessary to employ coatings to strip made prior to box annealing to act as a separator and avoid fusion of the adjacent strip surfaces at such high temperatures. The coatings conventionally employed are magnesia coatings such as are taught in US. Patent 2,385,332 to Carpenter et al. The magnesia coatings form a magnesia orthosilicate glass film on the surface of the strip which not only provides some insulation, but more importantly, provides an ideal base for subsequent phosphate coatings. It is the combined magnesia orthosilicatephosphate coatings that provide the excellent electrical insulating properties for the oriented silicon steel strip and sheet. However, non-oriented grades of silicon steel strip and sheet are employed for similar applications, but where orientation is not desired or required, are not subjected to high temperature box anneals but instead are conventionally heat treated by continuous annealing prac tices. Such practices do not require separating coatings, and the application of such coatings would be economically unfeasible to employ merely to obtain an adequate basis for employing the phosphate coats. Phosphate coatings for bare metal surfaces, such as are required in the production of non-oriented grades of silicon steel, have not proved to be too satisfactory. The application of a "ice - of a phosphoric acid plus silica to provide improved insulation; however, since it is necessary to maintain the silica in suspension by mechanical agitation, it is difficult to obtain a uniform and constant insulating coating, and additionally, such coatings do not provide as high a quality of insulating properties as the magnesia orthosilicate-phosphate coatings do for the oriented grades of silicon steel.

It has now been found that by employing colloidal silica in place of the milled mica conventionally employed for a phosphate-silica coating, a very superior insulating coating is obtained for standard grades of silicon steel strip or sheet. The coating provided by the method of the present invention gives greatly improved electrical insulation, and additionally is both adherent and water insoluble, which properties are requisites for transformer applications.

It is, therefore, the object of the present invention to provide a coating for electrical grades of silicon steel strip that will offer improved electrical insulation as compared to the prior known coatings.

It is also an object of the present invention to provide an improved method for applying an insulating phosphatesilicate coating to electrical grades of silicon steel strip.

Other objects and advantageous features will be ob vious from the following description:

In general, the present invention is directed to an improved method of providing a phosphate-silica insulating coating on fiat rolled electrical grades of silicon steel which comprises applying a slurry to the surface of said steel that contains phosphate compounds and colloidal silica, and subsequently baking the coated steel. The invention is particularly directed to the application of a slurry to silicon steel strip that is composed of from about 2% to 30% colloidal silica and 4% to 30% phosphates. The colloidal particles of silica may be of any size ranging in diameter from about one millimicron to 500, but being preferably of a particle size of from about 7 millimicrons to 100. The coated strip or sheet is preferably dried prior to baking, and baking is preferably conducted within the temperature range of from about 900 to 1400 F. It is also desirable to obtain maximum insulating properties while maintaining the desired and required space factor for laminated cores to maintain the dry weight thickness of the coating at from about .05 mil to .15 mil.

Conventional phosphate-silica slurries employed for the purpose of providing insulating coatings are generally composed of phosphate compounds such as phosphoric acid or ammonium phosphate. The end result is relatively independent of the exact compound employed, although some investigators express a preference of one source of phosphate to another. In any event, since hydrogen adds very little weight to phosphoric acid (hydrogen having an atomic weight of about 1.008 as compared to phosphorus at 30.97 and oxygen at 16), its presence may be ignored for practical purposes and it may be stated that the coating solution or slurry meets the requirements of the present ranges so long as there is present from about 6% to 10% of the phosphate ions, regardless of what specific phosphate compound or compounds are employed.

The conventional slurry also contains milled particles of silica ranging in size from about .5 micron to about 7 microns. The exact form of the milled silica has little effect on the insulating properties of the resulting coating. However, as in the case of phosphate compounds, specific materials such as diatomaceous earth or mica are said to be somewhat preferred. The conventional particles (.5 to 7 microns in diameter) are too large to form colloidal suspensions, and instead are maintained in solution by means of agitation. The silica employed in conjunction with the slurries of the present invention are only a minute fraction of the size of the milled particles. The colloidal particles may be as large as 500 miilimicrons, but if larger particles are employed it may not be possible to maintain them in a colloidal state, and consequently one does not enjoy some of the advantages of the present process by employing such a material. Colloidal silica is not obtained by milling conventional sources of SiO but is obtained by controlled condensation polymerization of silicic acids. Consequently, colloidal silica may have a particle diameter size as small as one millimicron, or as large as 500 millimicrons.

Colloidal silica is available commercially under the name of Ludox, and is a product of E. I. du Pont de Nemours & Co. This product is an aqueous solution containing about 30%, by weight, of colloidal silica. It is available in silica particle diameter sizes of 7 millirnicrons, 17 millimicrons, and 100 millimicrons. A more detailed description of Ludox may be found in U.S. Patent 2,809,137 to John Cornelius Robinson.

We have found that when the colloidal silica is used to make up the coating slurry in place of the milled silica, or of a silicate, such as is sometimes employed, a coating which exhibits superior insulation properties to the regular phosphate-silica when applied to bare metal surfaces may be effected. Such a slurry as employed by the method of the present invention is not only more easily applied, since the necessity of mechanical agitation is eliminated, but also provides more even and effective insulation. We have had particular success by employing slurries which contain from about 2% to 30% by weight of colloidal silica and 4% to 30% phosphoric acid, by weight.

The coating slurry or solution may be applied to the fiat rolled silicon steel strip surface in numerous ways, such as by painting or dipping so as to form a relatively uniform film of the material on the strip surface. However, conventional practice is to conduct silicon steel strip continuously through a tank containing the slurry or solution. Rubber metering rolls at the exit end of the tank are generally employed to meter the slurry onto the surface evenly. The wet film or coating is now conventionally permitted to dry, or is forcibly dried by blowing warm air onto the surface of the strip. The coating is then generally baked or cured by continuously heating the strip at temperatures as low as 700 F., but are conventionally continuous annealed at temperatures of up to 1400 F. Such treatment serves to cure or bake the insulating coating or render it insoluble. The drying step is, of course, not necessarily critical since the material would dry during subsequent curing or baking (particularly if this is done continuously). However, to obtain uniformity it is usually desirable to dry the film as soon as possible after the strip or sheet has left the coating tank.

The coating itself will, of course, be composed of phosphates and silica in approximately the same ratio as such materials exist in the slurry or solution. For example, steel coated while employing a slurry of 2 to 30% by weight of colloidal silica, or 4 to 30% by weight of phosphate, will have a dried or baked coating composed of from 6 to 88% by Weight of SiO;; and 12 to 94% phosphates.

The exact thickness of the coating will depend on the thickness of application, which is dependent on the gauge of the strip and the ultimate design application of the material; however, excellent results are obtained for coatings of about .05 to .15 mil in thickness, and the insulation properties are particularly desirable for coatings of approximately .10 mil in thickness.

In evaluating the insulating properties of coatings such as those applied in accordance with the present invention, electrodes are employed to contact both sides of the coated steel strip through which an electric current is passed, and a measure of the current and flux reflects the amount of resistivity offered by the coatings. The pressure applied by the electrodes is important, and in the present test was 900 pounds per square inch of electrode contacting rea. The electric current flow between the electrodes may be measured by an ammeter. For a detailed descrip tion of an appropriate measuring instrument see U.S. Patent 2,982,912 to L. T. Mitchell, Jr. At a .S-volt potential, the ammeter may be adjusted so that a reading of .00 represents substantially no flow of current, while a reading of 1.00 ampere would represent no insulation. Thus, the test results represent the fraction of one ampere of current permitted by the coating. Such measurements are commonly referred to, and will hereafter be called, Franklins.

In testing applicants coating, .018" gauge x 3" wide strip of standard electrical grade of silicon steel (02% C, 3.25% Si) was passed continuously through a coating tank. The coating solution or slurry in the tank was periodically changed so that the ultimate coil contained strip coated with various strength coating solutions. The coating was metered onto the bare surface of the strip by rubber metering rolls, and as the coated strip emerged from the metering rolls the coating was dried by conducting a stream of preheated air onto the coated surfaces. The strip was then continuously heat treated by passing it con tinuously through an annealing furnace which heated the strip to a temperature estimated between 900 F. and 1400 F. for a period of about one-half minute. The thickness of the coating was varied so as to be .05 mil or .10 mil.

Samples 3 x 8" were sheared from the various sections of the coated strip, and the insulation properties of the coatings were measured at Franklins in the manner described above. The results are shown in Tables I, II, III and IV below. The Franklin measurements are shown for each sample along with the coating solution composition (balance water). The Franklin measurements are shown for both the as-coated and as-stress relief annealed, 1550 F. in a cracked natural gas atmosphere, (about 7 parts air to 1 part cracked natural gas). The silica was present in each solution or slurry in colloidal form and both the silica and the phosphoric acid are reported as their weight percentage in water. The colloidal silica was added at Ludox (30% colloidal SiO in water with a particle size of about 17 millimicrons in diameter).

TABLE I All Tests H PO 6% As coated After 2 hr. anneal at 1550 F. in dry cracked gas Percent Coating Coating Coating 1 Coating Soln SiOg thickness, wt. Frankthickness, wt. Frank- N 0. using mils era/it. 1111 3 mils oz./ft. lin 3 Ludox 2 0.08 0.022 0. 29 0. 08 0. 025 0.71 2 0.05 0.022 0. 64 0.05 0. 020 0.97 4 0.10 0.051 0.05 0.10 0.008 U. 82 4 0.05 0.019 0. 21 0.05 0. 023 0. 93 6 0.10 0.020 0. 06 0.10 0. 024 0. 71 6 0.05 0.017 0.27 0. 05 0. 051 0.92 8 0. 10 0.020 0.06 0. 10 0.027 0. 54 8 0.05 0. 020 0.23 0.05 0. 024 0. 74 10 0.10 0. 023 0.07 0.10 0.030 0. 40 10 0.05 0. 024 0. 18 0.05 0.022 0. 92 15 0.10 0.026 0. 04 O. 10 0. 029 0.12 15 0.05 0.020 0. ll 0. 05 O. 023 0. 51 20 0. 15 0.026 0. 03 0. l5 0. 039 0. 03 20 0.10 0. 024 0.09 0.10 0.023 0. 54 25 0. 15 0.031 0.09 0. l0 0. 029 0.14 25 0.10 0.031 0.05 0.10 0. 022 0. 27

1 Measured with GE thickness meter Type B.

2 Coating removed with molten NaOH. Note: In some instancer not all of the coating was visibly removed after the prescribed %-hous treatment.

3 Average of top and bottom for pack of 8 samples.

5 TABLE 11 All Tests H PO 10% As coated After 2 hr. anneal at 1550 F. in dry cracked gas Percent Coating 1 Coating Coating Coating Soln. Si z thickness, wt., Frankthickness, wt., Frank- N 0. using mils oz./it. lin a mils oz./ft. lin 3 Ludox 1 Measured with G. E. thickness meter Type B. 2 Coating removed with molten NaOH. Note: In some instances not all of the coating was visibly removed after the prescribed %-hour treatmerit. J .r 3 Average of top and bottom for pack of 8 samples. g TABLE HI All Tests H,P0,-20%

After 2 hr. anneal As coated at 1550 F. in dry cracked gas Per- Time Coat. Coat.

Soln. cent reqd. Coat. wt., Frank- Coat. wt., Frank- No. SiOz to gel, thick, oz./ lin thick., oz./ lin using hrs. mils ft. mils it.

Ludox 0. 05 0. 032 0.85 0. 05 0. 044 0. 81 0. 10 0. 037 0. 77 0. l0 0. 042 0. 80 0. 15 0. 047 0. 07 0. 15 0. 046 0. 16 0. 10 0. 039 0. 51 0. 05 0. 043 0.42 0. O5 0. 038 0.81 0. 05 0. 040 0. 66 0. 05 0. 039 0. 84 0. 05 0. 040 0. 76 0. 10 0. 041 0.22 0. 10 0. 044 0. 28 0. 05 0. 035 0.89 0. 05 0. 041 0. 83 0. 05 0. 037 0. 78 0. 05 0. 041 0. 56 0. l5 0. 040 0. 44 0. 10 0. 044 0.30 0. 05 0. 032 0. 80 0. 05 0. 040 0. 57 0. 10 0. 040 0. 54 0. 05 0. 033 0. 29 0. 10 0. 041 0. 61 0. 10 0. 045 0. l5 0. 05 0. 057 0.44 0. 05 0. 049 0.09 25 Not feasible 0 mix 1 Measured with G.E. thickness meter Type B. 2 Coating removed with molten NaOH. Note: In some instances not all of the coating was visibly removed after the prescribed %-hour treatment.

8 Average of top and bottom for pack of 8 samples.

TABLE IV All Tests H PO As coated After 2 hr. anneal at 1550 F. in dry cracked gas Percent Coating 1 Coating Coating Coating Soln. SiOz thickness, wt., Frankthickness, wt., Frank- No. using mils oz./ft. lin 3 mils oz./ft. lin 3 Ludox 2 0. 05 0. 039 0. 80 0. 05 0. 039 0. 20 2 0. 15 0. 040 0.22 0. l0 0. 044 0. l3 4 0. 10 0.033 0. 15 0. l5 0. 050 0. 10 4 0. 05 0. 039 0. 63 0. 05 0. 045 0. 5O 4 0. 05 0. 021 0. 38 0. 05 0. 046 0.36 4 0. 15 0. 042 0. 07 0. 15 0. 046 0. 08 6 0. 15 0. 039 0. 06 0. 10 0.056 0. 10 6 0. 15 0. 035 0.47 0. 05 0. 046 0.30 8 0. l5 0. 050 0. 03 0. 40 0. 053 0. 05 8 0. 15 0. 038 0.24 O. 15 0. 049 0. 09 10 0. l5 0. 037 0.34 0. l5 0. 166 0. 25 10 0. l0 0. 038 0.45 0. 05 0. 168 0.26 15 0. 05 0. 045 0.27 0. 05 0. 051 0. 13 15 0. 05 0. 044 0. 40 0. 05 0. 048 0. 13

1 Measured with G.E. thickness meter Type B.

2 Coating removed with molten NaOH. Note: In some instances not all of the coating was visibly removed after the prescribed /g-hOlll treatment.

3 Average of top and bottom for pack of 8 samples.

The afore-mentioned data is particularly significant since .20 Franklin is considered to be acceptable insulation for the non-annealed material and .5 Franklin is acceptable for the as-stress annealed samples. The annealed material results are significant, since in most applications, to obtain the maximum magnetic and electrical properties it is necessary to stress relief anneal electrical grades of silicon steel strip to obtain the best electrical properties prior to its application. For example, as the strip or sheet is stamped into EI configuration for assembly into laminated cores, considerable stresses and strains are put on the metal which interfere with the electrical properties, and consequently the stamped lamination must be stress relief annealed prior to actual application. The effect of such an anneal on coatings is significant; it often destroys and usually causes deterioration of its electrical resistance properties. It should be noted by Table I that the coatings applied by slurries containing 6% phosphoric acid and 2 and 4% silica fail to meet minimum requirements for the as-coated samples where the coating thickness is less than .10 mil in thickness, and that all of these samples fail to meet the minimum requirements (.50 maximum) in the annealed condition. However, coatings evolved from slurries containing 6% phosphoric acid and 6 and 8% colloidal silica exhibit somewhat improved insulation properties, particularly in the as-coated condition, and where the coating is .10 mil in thickness. However, these coatings still fail to meet the requirements While in the annealed condition. Phosphate compositions of 6% with coloidal silica at 10% and 15% all provide excellent electrical resistance in the as-coated condition and provide adequate insulation properties in the annealed condition where .10 mil thickness coatings are applied. Where the silica content is at the 20 and 25% level, insulation properties in the as-coated samples are excellent, and are also excellent in the annealed condition for all but one test.

Table II shows results were 10% phosphoric acid was employed, and the colloidal silica was varied from 2 to 25%. These samples all show excellent electrical resistivity in the as-coated condition, whether the coatings are .05 mil in thickness or greater, and also show excellent insulation in the annealed condition, there being only one sample (containing 2% colloidal silica) which failed to meet the minimum requirements (0.50 Franklin).

It may be seen by Table III that where 20% phosphoric acid was employed the only samples that passed minimum requirements were those with .10 mil thick coatings which had been annealed. Such results indicate that it is desirable to employ greater than .05 mil thick coatings, particularly at the 20% phosphoric acid level, and that those coatings should be applied only where a stress relieving anneal is to be employed.

Table IV shows the results of employing 30% phosphoric acid plus various concentrations of silica. It may be observed that excellent results were obtained for the .05 mil thick coating, as Well as the heavier coatings, particularly in the annealed condition. It is to be noted that only one test reached the maximum allowability after annealing.

We claim:

1. The method of providing an inorganic insulating coating to fiat rolled electrical grades of silicon steel which comprises contacting the surface of said steel with an aqueous slurry that contains from about 2% to 30%, by weight, of silica having a particle size of from 1 to 500 millirnicrons diameter and a water soluble phosphatecontaining compound in such amount as to provide from 4 to 30%, by weight, of phosphate ions, so as to leave a thin coating of said slurry on said surface and baking said coating at a temperature of from about 900 F. to 1400 F.

2. The method of providing an inorganic insulating coating to fiat rolled electrical grades of silicon steel which comprises contacting the surface of said steel with an aqueous slurry which contains from about 2 to 30%, by weight, of colloidal silica having a particle size of from 7 to 100 millimicrons diameter and about 4 to 30%, by weight, of phosphate ions, so as to leave a coating of said slurry on said surface of from about .05 mil to .15 mil thickness when dried and baking said coating at a temperature of from about 900 F. to 1400 F.

3. A composition for the coating of electrical grades of fiat rolled silicon steel consisting essentially of an aqueous slurry containing from 2 to 30% of silica having a particle size of from about 1 to 500 millimicrons diameter, 4 to 30% phosphate ions and the balance water.

4. The method of providing an insulating coating to flat rolled electrical grades of silicon steel which comprises contacting the surface of said steel with an aqueous slurry that contains from about 2% to 30%, by weight, colloidal silica having a particle size of from 1 to 500 millimicrons diameter and 4% to 30%, by weight, phosphate ions, so as to leave a thin coating of said slurry on said surface, drying and baking said coating.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE METHOD OF PROVIDING AN INORGANIC INSULATING COATING TO FLAT ROLLED ELECTRICAL GRADES OF SILICON STEEL WHICH COMPRISES CONTACTING THE SURFACE OF SAID STEEL WITH AN AQUEOUS SLURRY THAT CONTAINS FROM ABOUT 2% TO 30%, BY WEIGHT, OF SILICA HAVING A PARTICLE SIZE OF FROM 1 TO 500 MILLIMICRONS DIAMETER AND A WATER SOLUBLE PHOSPHATECONTAINING COMPOUND IN SUCH AMOUNT AS TO PROVIDE FROM 4 TO 30%, BY WEIGHT, OF PHOSPHATE IONS, SO AS TO LEAVE A THIN COATING OF SAID SLURRY ON SAID SURFACE AND BAKING SAID COATING AT A TEMPERATURE OF FROM ABOUT 900*F. TO 1400*F. 